An increasing number of mobile applications use a radio-frequency communication system to exchange data between different nodes. The production constraints are often low consumption constraints, especially when power is provided by batteries or systems for recovering ambient energy.
Because of the increasing number of applications, the UHF frequency bands around 2.4 GHz that are often employed by these systems, notably for industrial, scientific and medical applications, are more and more congested, thus increasing the number of interfering signals liable to saturate the input of the receiver. To provide protection against such interference, the designer of the system has the option either to increase the linearity of the receiver, generally at the cost of an increase in its electrical consumption, or to increase its selectivity, ideally a long way upstream in the reception chain and thus as close as possible to the antenna. This second approach thus creates a requirement for band-pass filters centered on UHF frequencies, having a relatively narrow bandwidth (of the order of the width reserved for a channel in the authorized frequency distribution), and able to be incorporated into the receiver with minimum impact on its electrical consumption.
The necessity to work directly at UHF frequencies rules out the use of active filtering techniques typically used at lower frequencies. The aim is therefore to use passive resonators operating directly at UHF frequencies, based for example on Surface Acoustic Wave (SAW) or Bulk Acoustic Wave (BAW) technologies, i.e. surface acoustic wave resonators and bulk acoustic wave resonators.
Such resonators may be used as individual resonators associated with an active circuit which thus benefits from the frequency selectivity of the resonator around its resonant frequency or antiresonant frequency, or as a set of individual resonators separated from each other by active elements, or as resonators in a purely passive filter itself associated with an input circuit and an output circuit, each of which two circuits may be active or passive. The filter functions obtained with individual resonators have limited selectivity and do not always provide a sufficiently flat bandwidth in the useful band. For their part, active circuits cause the structure to consume more energy, which is undesirable.
In the prior art, passive filters have already been proposed based on one or more lattice cells (known as “lattice filters”) with differential input and output, each cell employing four resonators connected in “direct” and “crossed” manner between the input and the output of the cell. Two types of resonator are conventionally used to produce “series” or “parallel” resonators. Each resonator has a characteristic impedance Zc, a resonant frequency (Fr) and an antiresonant frequency (Fa), where Fa>Fr. The resonant frequencies of one type of resonator are different from those of the other type, and the same applies to the antiresonant frequencies. The resonant frequency of the resonators of a first type is conventionally made equal to the antiresonant frequency of the second type, to obtain a bandwidth sufficiently flat around the center frequency of the band.
These lattice filters are placed between an input circuit and an output circuit, and it is generally accepted that the input circuit impedance is matched to the impedance of the filter as seen from its input when it is loaded by an output circuit, and likewise the output circuit impedance is matched to the impedance of the filter as seen from its output when its inputs are connected to the input circuit; the purpose of this matching is to optimize signal power transfer.
The input and output impedances of the filter and the characteristic impedance of each of the resonators are most often chosen to have a common standard value. The common standard values are most often 50 ohms, 100 ohms and possibly 200 ohms (notably for differential operation).
One example of a resonator particularly suitable for radio-frequency transmission systems is the Bulk Acoustic Wave (BAW) resonator enabling resonance to be obtained at frequencies of the order of 800 MHz to 3 GHz corresponding to the UHF bands routinely employed in modern telecommunication systems. The resonator is constituted, for example, by a substrate recessed on the rear face, a support membrane over the recess, and a layer of piezoelectric material sandwiched between two metal electrodes. The resonant frequency of this resonator may be adjusted by loading the stack of layers with an additional layer that adds mass to the stack and thus reduces the resonant and antiresonant frequencies.
These resonators have a high quality factor and are of very small size. They are not easy to use, however, when there are required simultaneously a very low bandwidth, a band that is flat in the wanted band, and low consumption of the filter structure that incorporates the resonators.